just that it would be something that some collectors do not like to see, and that hurts the value.
But not that much, that the watch is now not worth collecting. It is just that it is obvious that a part of the watch was replaced with a newer part, and that is never adding value...
I agree with Marcello that it is a case of finding the right back, and then selling the present one. I would think that the difference in value of the backs, when found, can be 500-1000 euro depending on condition.